The quantitative and qualitative analysis of gases and their mixtures has been found to be vastly applied in the fields of global environment monitoring, household safety inspecting, greenhouse environmental control, chemical concentration control, and certain applications relating to aerospace industry, etc. Nowadays, it is common to use gas sensors for performing the quantitative and qualitative analysis of gases and their mixtures, since not only the cost of monitoring the gases and their mixtures and the testing cycles required to be performed in the monitoring can be reduced, but also a real-time monitoring of the gases and their mixtures can be achieved thereby. However, cross sensitivity problem is common to those currently available gas sensors, such as semiconductor oxide gas sensors, metal oxide gas sensors, electrochemical gas sensors and solid electrolyte gas sensors, which can cause the reliability and repeatability of a monitoring result performed by such gas sensor to be adversely affected, i.e. the aforesaid gas sensors will fail to measure the individual concentration of each target gases of the monitoring accurately. Although, a gas sensor consists of an array of sensors sensitive to different gases can be used for detecting and measuring a plurality of gases, the cross sensitivity problem still can not be eliminated. For solving the foregoing cross sensitivity problem, the ability of certain gases to absorb infrared radiation has been successfully utilized in developing optical instruments for gas sensing, that is, gases can be selectively detected by the utilization of an infrared sensor via their specific absorption in the infrared spectral range. Despite their functional superiority, the optical gas sensors were not initially popular due to their structural complexity and high manufacturing cost, especially as the size of the optical gas sensor is increasing with the increasing of the amount of optical parts and relating elements of the optical gas sensor needed for detecting and measuring a plurality of gases. Therefore, the optical gas sensor currently available can only be used to detect and measure a gas of the specific infrared spectral range of the gas sensor, that the optical gas sensor can not be adaptively controlled for detecting and measuring various harmful gases coexisted in a same environment.
Please refer to FIG. 1, which is a schematic illustration of a conventional optical gas sensor used for detecting a specific gas. The optical gas sensor 1 of FIG. 1 is comprised of an infrared radiation source 10, a reference light source 11, a chamber 12, a narrow-band optical filter 13 and a photodiode 14, wherein the reference light source 11 is disposed in the chamber 12 intermediate to the first and second ends of the chamber 12, that is, at a distance from the photodiode 14 less than the distance between infrared radiation source 10 and photodiode 14, and the narrow band optical filter 13 selected for a specific wavelength with respect to a gas to be sensed is mounted between the reference light source 11 and the photodiode 14. As the infrared radiation source 10 is emitting light of a defined wavelength range to be transmitted and reflected in the chamber 12, the gas to be sensed in the chamber 12 will absorb the emitted light while enabling the absorbed light of the specific wavelength to pass through the narrow band optical filter 13 to be received by the photodiode 14. Since the light of the specific wavelength emitted by the reference light source 11 is received by the photodiode 14 without having to travel across the chamber 12 filled of gas to be sensed and thus it is not subject to the absorption of the gas to be sensed, the gas to be sensed can be detected and the concentration of the same can be measured by comparing of the intensity of the light emitted form the reference light source 11, which is used as a reference value or initial value, with that of the light emitted from the infrared radiation source 10 after passing through the chamber 12. However, the use of the reference light source in this basic optical gas sensor configuration is to compensate for changes and deterioration of optical components with time and temperature. In practice, the reference light source is added to the sensor to correct for these potential problems.
There are many optical gas sensors currently available, such as those disclosed in U.S. Pat. No. 6,067,840, U.S. Pat. No. 6,469,303, U.S. Pat. No. 6,392,234, U.S. Pat. No. 5,610,400, and U.S. Pat. No. 5,550,375. It is noted that those shown in U.S. Pat. No. 6,067,840, U.S. Pat. No. 6,469,303, U.S. Pat. No. 6,392,234, U.S. Pat. No. 5,610,400, and U.S. Pat. No. 5,550,375 are only suitable for detecting a specific gas while the reference light source for emitting reference light and the infrared radiation source for emitting testing light used in the device shown in U.S. Pat. No. 6,067,840 are two different light sources.
From the above description, there are four major shortcomings can be summed up as following:                (1) By having reference light and testing light to be emitted from two different light sources as those used in prior-art sensors, it is possible that one might not be able to distinct the initial value, being obtained from the reference light representing no target gas sensed, from a response value, being obtained from the testing light representing the existence of the target gas, since the two light sources might begin to deteriorate at different times. Therefore, it is preferred to have the reference light and the testing light to be emitted from a same light source so that the time of deterioration of the two is identical and thus the distinction between the initial value and the response value is ease to identify.        (2) It is known that the reflection index of a material/atmosphere is varying along the change of ambient temperature, pressure or the properties of the material, and the change of reflection index will consequently cause the corresponding optical path to change. Hence, since the length of the optical path of the reference light is different from that of the testing light as those used in prior-art sensors while the initial value is subject to the influence of ambient temperature, pressure and the properties of the material, the accuracy and long-term stability of the gas sensor are reduced.        (3) The prior-art gas sensors can not be adapted for multi-gas testing.        (4) The structure of the prior-art gas sensor can not be flattened.        
Therefore, it is in great need to have an apparatus for sensing plural gases that is capable of overcoming the foregoing problems.